What's an "OWS"?

OWS is an abbreviation for Overload Warning System. An overload is an "over G" condition. Just as the ONSPEED aural AOA logic is designed to act like a flight instructor and tell the pilot how hard to pull on the pole to avoid the aerodynamic (stall) limit of the flight envelope, the OWS logic performs the same function for the G limit of the airplane.


Over G warning in electronic flight information systems is nothing new. In the V3, we simply use the on-board inertial reference unit (IRU) to measure G and let the pilot know when the limit is reached. Unlike the ONSPEED logic, we decided not to use progressive warning; but simply provide a simple "G Limit" voice warning when the limit is reached. In the F-15, the OWS provided progressive warning--but it worked in a manor very similar to the AOA logic and there is such a thing as too many warning tones! Since we generally don't have a 1:1 thrust to weight ratio and don't have to hit a perfect, symmetric 9 G's in our EAB airplanes, we figured a friendly reminder to quit pulling so hard was sufficient.


Because the OWS uses the IRU, it not only knows how many G's you're pulling; but it knows whether or not the airplane is rolling. This is handy since the G limit for the airplane is less if it's rolling. This is because the G load on the wings isn't symmetric when the airplane is rolling. Most of the time, manufacturers don't specify "asymmetric" (rolling G) limits. Unfortunately, the design margin for rolling G limits generally aren't taught or well understood, unlike the margin for ultimate G limit.


Ultimate G Limit. If you recall from pilot training, the engineers design a 50% margin into the airplane; so the wing on the 2.5G limited 777 I fly at work will fail at 3.75 G's (in static testing it actually failed at 3.76, so there definitely isn't an extra pound of margin!). You'll also recall that if you exceed the G limit, you may damage the structure, even if you don't reach the ultimate (failure) limit. And, of course, recall that metal has a memory...bend it enough times and it will break. In engineering terms, this is referred to as "fatigue failure." Exceeding G limits simply isn't good for the airplane.


Asymmetric G Limit. Just like the 50% margin for ultimate G limit, there is a margin for asymmetric G limit. For civilian airplanes, FAR 23 specifies 33%. In other words, if the airplane is in aerobatic category and is rated to 6 G's, the asymmetric limits is 33% less, or 4G's. Now some manufacturer's get around this by building the structure to accommodate full G limits while rolling (the Extra 300, for example). These are typically purpose build aerobatic aircraft that need lots of structural strength for snap maneuvering. So, unless the pilot's handbook or designer's specifications state otherwise, it's assumed that whether specified or not, rolling G limits are 33% less than regular G limits.


Flaps Down G Limit. With flaps deployed, FAR 23 specifies a 2G limit.


The neat thing about the OWS, is all of this is transparent to the pilot. It just tells you when you are at a limit and need to quit pulling any harder. It knows if you are rolling or not, and, if equipped, knows where the flaps are. As a matter of fact, the roll rate to "trip" asymmetric logic is selectable in the the code. Fighters use roll rates in excess of 45 deg/second to signal asymmetric maneuvering limits; but I flew a lot of bent fighters that had straps riveted on to the wings because of abuse. Because of this, we selected some pretty conservative numbers to trigger asymmetric warning: I don't want to bend my RV-4. And if I can't fly a maneuver with 4G's or less, I'm probably boning it up anyway...


To program the OWS, the pilot selects the aircraft category using the WiFi interface and the software logic takes care of the rest, no matter how bad a day the stick monkey is having. We've also added a training mode with a 2.5G limit. This allows the logic to be used during upset training. 2.5G's was not an arbitrary number. First, it's obtainable in any Normal, Utility or Aerobatic category light plane, and second it mirrors FAR25 requirements for heavies, so the mode is helpful to calibrate the pilot's internal IRU (i.e., butt) if the mode is used when introducing folks to upset and unusual attitude training.


Airspeed Warning. Because there is one more side to the flight envelope, we've also included a simple airspeed warning. Again, because of my background trying to max perform the airplane all of the time, we deviated slightly from normal civilian logic. We actually provide a gentle chime when the airplane reaches Vno, or maximum structural cruising speed. This is the top of the green arc on a conventionally marked ASI. The reason for this is two-fold. First, Vno is an indicated airspeed (unlike Vne in some airplanes); and second, it is a structural limit. It's OK to fly in the yellow arc in smooth air; but it needs to be a conscious decision by the pilot to do so. Hence the chime logic, which tells the pilot that you are going as fast as you want to within limits and then gently asks, do you want to exceed the structural limit? The chime interval is pilot selectable via the WiFi interface, so for folks that don't engage in aerobatics or dog fighting, a longer interval would be appropriate (say every 5 or 10 seconds). And, like all features, it can be turned off if you like to dive down at redline as a matter of course.

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